I'm using two HP 34401A multimeters. I configured one as a voltmeter and the other one as an ohmmeter. I connected the positive leads together and the negative leads together. The voltmeter reads 2.6 V. I tried this with a Fluke multimeter, and again it is 2.6 V, so I assume this is some kind of standard. Is the ohmmeter really applying 2.6 V between its leads? If so, this can easily destroy many of the low voltage chips I have.
frank
Hello Frank,
Yes, it can. Know thy tools ;-)
-- Regards, Joerg http://www.analogconsultants.com
Actually I don't think the ohmmeter applies 2.6 V, because if I attach an 1 Kohm resistor across the ohmmeter's leads and measure the voltage across it, the voltage is only 1 V. This implies the ohmmeter actually applies a current through the resistor, in this case 1mA. This current depends on the range. Of course, the question now it that can 1 mA kill my circuit? ;-D
frank
Geneally not. Most any volt-ohm meter today uses safe area voltages and currents. Additionally, many newer digital multimeters use very low voltages for the ohms circuit to avoid reading thru diode type internals of ic's. These same meters often provide a 'diode' type mode that deliberately uses a higher voltage to test diode conduction.
Luhan
Hello Frank,
Still, if no resistor is connected and it reads 2.6V then that would be the exposure. For a hypothetical example consider a really hot RF MOSFET where it's gate could only tolerate 2V. It is hi-Z so it doesn't present much load to the ohmmeter. Even if there were a large resistor bias circuit that might not drop the ohmmeter voltage much if the meter was switched to a lower range. Chance are that this would send the RF MOSFET into lalaland.
With most chips there will be parasitic substrate diodes. While their capacitance contribution is often undesirable they do provide a safety net. Unless the ohmmeter is switched to a very low range chances are they might be capable of handling the current and divert it towards one of the power or GND rails. Note that this is not the case for some FET switches. A wee exceeding of the limits and bzzzt.
-- Regards, Joerg http://www.analogconsultants.com
Hi Joerg, Hi Luhan,
Thanks for your prompt and insightful answers. It seems it is "dangerous" to do some very basic DC debugging using these popular multimeters. I can't use the ohmmeter on low voltage circuits. I can't use the "continuity" mode because it also applies voltage between two nodes (~7 V). So unless I'm absolutely sure in the first place that the two nodes are shorted I can't touch those two nodes. Also, Luhan, could you suggest some of these newer multi-meters (make and model) designed especially for low voltage device testing?
Thanks,
frank p.s. How does the continuity mode work on these multimeters?
"frank" wrote in news: snipped-for-privacy@m79g2000cwm.googlegroups.com:
Ohms mode is a constant-current source with a open Vmax of 2.6V as you measured. Higher ranges may have even higher voltages.You need to check your meter's manual.
Your meter measures resistance by passing a known current through them and measuring the voltage across the unknown resistance.
-- Jim Yanik jyanik at kua.net
Hello Frank,
It's all different. Best to obtain a detailed data sheet which should state open circuit voltages and max currents. Mine go to the 200ohm range for continuity (meaning a rather high current), the beeper is turned on and there is an indicator for diode drop so you can figure out whether it's a Schottky diode or a regular one. I don't use it for Ge small signal diodes because it may be too much for these.
-- Regards, Joerg http://www.analogconsultants.com
Hello Jim,
It depends on the meter. An el-cheapo dual-slope digital meter doesn't necessarily maintain constant current.
-- Regards, Joerg http://www.analogconsultants.com
Most of them are not exactly constant current. They use a differential reference input that compensates for the change in current through the unknown resistance. That makes a lot of errors cancel out.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
You may want to consult the manual. My Fluke model 87 came with a manual that explained everything in great detail. When in doubt RTFM!
Chris
Yes, and no. Yes: you are reading 2.6V from the ohmmeter; in that case the load is
10 megohms (the DVM). NO: the ohmmeter *cannot* easily destroy anything you may wish to measure as the Device Under Test will have a much lower resistance and the voltage will be much lower (*and* the current will also be quite low). Notice that a diode in forward direction measures roughly 0.560 on the ohmmeter; the voltage across it will read 0.560V, and the current will probably be 1mA (approx guess).
My Fluke 75 puts out about 0.1 volt into 1 megohm in the resistance mode. Even that voltage drops off rapidly with lower ohms loads.
It puts out 2.5 volts in the diode/continuity test mode.
I have never had an instance of damaged parts using this meter over the last 30 (?) years . Nor have I ever heard of such damage from anyone else. Its hard to find any component that wont handle 10 ma. That is what the protection diodes in many IC's are rated at. My fluke works out to (0.1/1000000) = 0.1 microamps.
Luhan
Joerg wrote in news:7gMIg.12740$ snipped-for-privacy@newssvr27.news.prodigy.net:
HP34401A is not a cheapo meter. Sure,it only cost $995 originally,but is a accurate quality 6.5 digit 4- terminal-measurement DMM.
-- Jim Yanik jyanik at kua.net
Hello Frank,
I was just mentioning this in case newcomers happen upon this thread. So they know that things might not be so clear cut for that meter they picked up at the hardware discounter or the $19.95 edition from the gas station.
Once I had a hard time resisting the temptation to buy this HP meter. I didn't really need one but it certainly caused some drooling.
-- Regards, Joerg http://www.analogconsultants.com
In message , dated Tue,
29 Aug 2006, Joerg writesMany of these are in fact harmless. The real risk is with legacy meters. The AVO 7 has a 10 mA movement, so it can supply at least 10 mA. My first employer found that it wasn't a good meter to use for checking extremely costly 10 mA fuses! On the low-ohms range it can supply more than 100 mA, I believe.
-- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
I like mine.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Hello John,
One of the Russian meters could make a flashlight bulb glow. I got a hunch this would happen when I found that it needed D-cells.
-- Regards, Joerg http://www.analogconsultants.com
Hello Spehro,
< drool... >Why did you have to do that? It's like blowing cigar smoke into the face of someone who just quit smoking ;-)
-- Regards, Joerg http://www.analogconsultants.com
Hello Robert,
Be careful with really hot RF transistors. IIRC it takes a mere 1V of reverse bias for a BFP520 and bzzzt. Kaputt.
There won't be any current. Well, after the moment of bzzzt I guess that changes ;-)
-- Regards, Joerg http://www.analogconsultants.com
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